Self-stacking spiral conveyors employ a pervious conveyor belt for conveying products in a spiral or helical path through a cooking, drying, cooling, or freezing chamber. A heat transfer system and other applicable systems provide the appropriate gas treatment within the chamber for cooking, drying, cooling, or freezing products. These spiral stacks or conveyors generally have space efficiency in that they have a small footprint while providing a relatively long processing path. However, it is often a challenge to direct the flow of treatment gas within the chamber to evenly and completely cook, dry, freeze, or refrigerate the products.
In a typical spiral stack, treatment gas is directed vertically downward or upward within the stack. Such a vertical flow pattern often causes uneven temperatures within the stack because the products are more densely packed on the interior side of the conveyor belt. A crowded configuration results because the products are loaded evenly on a linear portion of the conveyor belt at the entry of the freezer system, and the belt collapses on the interior portion (or expands on the exterior portion) when the belt changes to a circular shape as it enters the spiral path of the stack. The products on the interior portion of the conveyor belt are moved closer together, and/or the products on the exterior portion of the conveyor belt are moved farther apart from one another, resulting in an uneven distribution of products on the conveyor belt.
With substantially vertical airflow through the stack, the products on the interior portion of the conveyor belt will not be treated to the same degree as the products on the exterior portion of the conveyor belt. In a specific example of a spiral freezer, the products on the interior portion of the conveyor belt will not be frozen to the same degree as the products on the exterior portion of the conveyor belt for at least two reasons. First, heat from the crowded products increases the temperature of the treatment gas on the interior portion of the stack as the gas passes vertically over the products. Thus, the products on the interior portion of the belt are exposed to warmer treatment gas for heat exchange. Second, the more densely packed products on the interior portion of the belt restrict the vertical flow of the treatment gas. Therefore, the products on the interior portion of the belt are also not exposed to a sufficient amount of treatment gas for heat exchange.
An existing solution to this problem is to perforate the inner side links of the conveyor belt to allow gas to flow radially across the stack while positioning a horizontal partition or sheeting within the interior of the stack to direct radial flow. Moreover, in a vertical downward flow system, the internal sheeting is positioned lower than an outer mezzanine extending between an exterior of the stack and an interior of the freezer chamber (and the internal sheeting is positioned above an outer mezzanine in a vertical upward flow system). As a result, a more continuous flow of colder treatment gas (substantially unaffected by product heat) is directed radially through the inner side links of the belt and across the stack. This configuration is described in more detail below with reference to the prior art system depicted in FIG. 1.
A drawback to the above-described solution is that the vertical position of the internal sheeting is fixed, and the position is not always optimal for every freezer system. For instance, the optimal placement of the internal sheeting may depend on the type of product to be frozen, the variables of the treatment gas (type, velocity, temperature, moisture content, etc.), the direction of vertical flow, or other variables. Calculations could be made to predict the optimal placement for the internal sheeting, however, such calculations are time consuming, inaccurate, and dependent on many variables. The internal sheeting could also be made adjustable in height, however, such a solution is mechanically difficult to implement and expensive.
Moreover, even with optimal placement of the internal sheeting, uneven treatment of the products may result. The above-described solution increases air flow to the interior of the stack, however, the increased air flow may be insufficient. As a result, the products positioned on the interior portion of the conveyor belt may be warmer than the products on the exterior portion of the belt. If the products are arranged in rows across the belt, and the products are mixed when packed, the uneven temperature of the products may even out when the mixed products are packed together. However, if the products are instead packed by rows, the packed products from a row on the interior portion of the belt may be too warm and inadequately frozen. Likewise, packed products from a row on the exterior portion of the belt may be too cold, possibly resulting in a brittle product that could break during packaging.
In a converse example, the above-described solution may actually overcompensate and cause excessive air to flow through the interior of the stack. As a result, products on the exterior portion of the conveyor belt are warmer compared to the products on the interior portion of the belt. If the products are packed by rows, the packed products from a row on the exterior portion of the belt may be too warm, and packed products from a row on the interior portion of the belt may be too cold.
Other possible alternative solutions include loading products less densely on the interior side of the belt (or more densely on the exterior side) at the entry of the freezer system such that the products are more evenly distributed when the belt collapses on the interior portion (or expands on the exterior portion). However, loading the products onto the belt in this manner is difficult to manage, inconsistent, and it may decrease the output capacity of the spiral stack. As yet another alternative, the degree of perforation in the inner side links of the conveyor belt may be adjusted or optimized for every freezer system. However, such a solution is unrealistic and cost prohibitive.
Thus, there exists a need for a gas circulation system for a self-stacking spiral conveyor that can be adjusted to create optimal gas flow treatment for a specific product and that improves treatment results and maintains output product yield from the system.